We have been investigating genes in Drosophila melanogaster that are required for accurate chromosome segregation during mitosis. Mutations in three of the genes we are studying result in the appearance of highly polyploid cells sometimes containing hundreds of chromosomes. Polyploid anaphase figures are observed, suggesting that the products of these genes function in a post-anaphase event we believe to be cytokinesis. The genomic regions containing all three genes have been cloned; we propose to continue these investigations so as to identify and to sequence the proper transcriptional units, and to generate antibodies against the corresponding gene products for future immunolocalization studies. A strong effort will be made to advance our understanding of the role of a fourth gene, l(1)zw10, mutations in which cause aneuploidy in a large proportion of cells in many types of mitotic tissues. Our cytological characterization of mutants suggests that the l(1)zw10 protein is required for proper sister chromatid separation at anaphase onset. We have further shown that the intracellular location of the protein encoded by l(1)zw10 undergoes several remarkable transitions in the course of the cell cycle. In particular, it resides on or near kinetochore microtubules at metaphase, but is then rapidly redistributed to the kinetochores at the beginning of anaphase. The experiments outlined in this proposal are designed to explore possible function of the l(1)zw10 product. We will ask whether this protein is required for any chromosome movements to the metaphase plate, determine whether it participates in the system rendering anaphase onset dependent on spindle integrity, and further investigate puzzling aspects of the phenotype that may provide clues about its functional role. We also intend to study the structures in which the protein is found at the ultrastructural level, and to determine how the novel intracellular distributions of the protein to these structures are achieved. Examination of the effects of other mitotic and meiotic mutations on the localization of the l(1)zw10 antigen may illuminate gene interactions required both for the localization pattern as well as other aspects of chromosome segregation. Finally, by exploiting the reagents we have generated, other components of the important mitotic organelles in which the l(1)zw10 product reside will be identified.